The present invention relates to a process for producing a wound capacitor of capacity C, in which process at least two films of metal coated dielectric material are wound up one on the other, the metal layers forming the plates of said capacitor, the instantaneous capacity Cx of the capacitor being measured at all times by means of capacity measuring means.
Different methods are known for producing wound capacitors from metal coated dielectric films.
A first method consists in counting the number of turns formed and in stopping the machine after a predeterminded number of turns. However, such a method has proved very inaccurate giving rise to a very great dispersion in the capacities of the capacitors produced. Such a method is only generally suitable for very high capacities for which the relative accuracy of the value of the capacity remains nevertheless acceptable.
Another known method consists in winding simultaneously the two metal coated dielectric films with a machine comprising a capacity bridge which follows the evolution of the capacity of the capacitor thus wound and stops the machine when the desired value is reached. However, with this method very high capacities cannot be wound in a single operation for then the resistance of the metal coating becomes too high and thus falsifies the measurement of the value of the capacity. This problem has been solved in practice by proceeding in several successive steps: for example, to produce a capacitor having a capacity of 10 microfarads, the measuring bridge is adjusted to 2 microfarads, a first winding of 2 microfarads is made and then the metal layer is removed from one of the plates so as to isolate the beginning of this winding from the rest of the capacitor. Then a new winding of 2 microfarads is formed on the first one, and so on until the desired value is obtained. Thus, the resistance due to the metal layers is much smaller and an improvement is obtained in the relative accuracy of the capacitors thus obtained. It should in fact be stated that the measurement of the capacity during winding cannot of course be effected at the lateral Schooping connections as is the case with a finished capacitor, since they have not yet been formed at this stage of the process. Consequently, the measurement of the capacity is generally effected by means of two conducting rollers disposed in contact with the metal layers of each of the two dielectric films just before winding thereof. Consequently, the resistance of each metal layer has considerable influence since a part of the AC current used for measuring said capacity is propagated as far as the end of each metal coated dielectric film which may have a length of several hundreds of meters.
The second method described above does not however give entire satisfaction for small capacity values, for the capacity variation per unit of length is in this case high, whereas the mechanical accuracy of the machine is imperfect (by capacity variation per unit of length is to be understood the capacity variation per unit of length of wound film). These two factors cause then a not inconsiderable dispersion of the values of the capacities of capacitors thus produced.